Semiconductor integrated circuits typically have thousands of devices, such as field effect transistors with source/drain regions and a gate electrode, which must be electrically contacted. The transistors are frequently contacted through openings or windows in a patterned dielectric which expose at least portions of the source/drain regions or gate electrode. Additionally, electrical connections between devices must be made, e.g., from the gate of one transistor to the source/drain region of another transistor or between source/drain regions of different nearby transistors. These connections are frequently referred to as local interconnects and often run over intervening features such as runners. The local interconnects, which may be formed of polysilicon or titanium nitride (TIN), are electrically isolated from the underlying features by a dielectric layer.
The metal of choice for many integrated circuit applications has been aluminum. This metal is relatively easy to deposit and then pattern by etching, and has a relatively high electrical conductivity. However, aluminum exhibits drawbacks at submicron dimensions for local interconnects. Typically, processes that use local interconnects subject devices to temperatures greater than 800.degree. C. after the local interconnect has been formed. This temperature makes it virtually impossible to use aluminum as a local interconnect. Further, aluminum interconnects are prone to failure because of, e.g., electromigration.
Accordingly, alternatives to aluminum have been sought. One extensively investigated alternative to aluminum is formed by the group of metallic silicides such as titanium or tantalum silicide. The silicon to metal ratio of deposited silicides is very hard to control and makes deposited silicides unsuitable for manufacture. Additionally, deposited silicides are not self- aligned. Alternatively, silicides are frequently formed by depositing a metal layer on silicon and heating the two materials so that they react to form the desired silicide. The reaction rate between many metals and silicon is reasonably fast and controlled at silicide formation temperatures. These temperatures are compatible with the integrated circuit processing sequence. Unreacted metal, if any, may be removed after heating is completed. This process is conceptually simple but requires appropriate amounts of metal and silicon so that a silicide is formed everywhere. Silicides are frequently used to increase the conductivity of polysilicon features, such as runners or local interconnects, because the silicide has a higher conductivity than does the polysilicon.
However, problems may arise in the implementation of the silicide forming, i.e., siliciding, process described. This is especially true when silicided local interconnects are formed over relatively closely spaced features. For example, a layer of silicide forming metal such as titanium may be deposited after polysilicon has been deposited and patterned to form conductive features. However, sputtered titanium does not deposit conformally on the polysilicon and the titanium thins at the bottom edges of the region between the features due to shadowing. The lack of metal in this area results in very little silicide formation at the bottom edges. Agglomeration of thin silicide at the bottom edges of runners can lead to undesirably high resistances or, in the worst case, an open circuit.
The properties of titanium silicides are discussed in the literature. See, for example, Applied Physics Letters, 48, pp. 1591-1593, Jun. 9, 1986 and Journal of Applied Physics, 57, pp. 5240-5245, Jun. 15, 1985.